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LONDON, Ont. -- Some Ontario doctors have ignored warnings about mixing two common drugs and now London researchers say they know the toll -- hundreds of seniors needlessly suffering kidney failure and some even death.

Lipitor is the most prescribed drug in Canada and research has shown it reduces heart attacks and death. Clarithromycin and erythromycin are antibiotics used to fight sinus infection, pneumonia and other respiratory infections. But the two combined can be life-threatening.

That risk has long been noted among many dangerous drug interactions doctors and pharmacists are supposed to heed.

But tens of thousands of Ontario seniors were given both anyway and hundreds were hospitalized with kidney failure from 2003 to 2010, a new London study shows.

"We're hoping physicians and pharmacists will take more care," said Dr. Amit Garg, whose study was published this week in the Annals of Internal Medicine.

Previous warnings were based on studies of the smallest scale in which a dozen or two healthy volunteers were given both statins like Lipitor and their blood was tested.

But the London study looked at 75,000 Ontario seniors who got both drugs between 2003 and 2010. Within 30 days of getting the antibiotics, hundreds suffered kidney failure and were hospitalized and more died than a control group that didn't get both drugs.

"It's preventable," said Dr. Amit Patel, an internal medicine resident at London Health Sciences Centre and co-author of the study.

That sort of real-world impact should persuade doctors to avoid prescribing both drugs, said Garg, a researcher at Lawson Health Research Institute, a kidney specialist at the London Health Sciences Centre and a professor of epidemiology and biostatistics at the Schulich School of Medicine & Dentistry.

"Until now, the clinical and population-based consequences of this potential drug-drug interaction were unknown," Garg said.

Physicians should look at alternatives when co-prescribing these combinations of drugs: